Carbon film resistor composition



Sept 19, 1967 MITSUO WADA 3,342,752

CARBON FILM RESISTOR COMPOSITION Filed Sept. 2, 1965 6Q B X P76 3 5% m w 2 H 2 Q Q 3 Q, R b E Q 50 k s S k L Q 1) K) I m 15 0 W. n 0 500 /000 L500 20w Q 79577779 #2779 m hour INVENTOR l l'l csuo Wade ATTORNEYS United States Patent Ofiice 3,342,752 Patented Sept. 19, 1967 3,342,752 CARBON FILM RESISTOR COMPOSITION Mitsuo Wada, Moriguchi-shi, Japan, assignor to Matsushita Electric Industrial Co. Ltd., Osaka, Japan, a corporation of Japan Filed Sept. 2, 1965, Ser. No. 484,663 8 Claims. (Cl. 252-511) ABSTRACT OF THE DISCLOSURE Carbon film resistors having substantially increased stability under extreme humidity conditions are prepared by coating a suitable ceramic base with a composition consisting essentially of finely divided carbon and a finely divided reducing metal oxide in a suitable resin base, and curing the composition. The reductible oxide is selected from the group consisting of P Mn O Cu O and V 0 and mixtures thereof, and the proportions of metal oxide to carbon are preferably about 1.5-12. The resin base preferably constitutes about -65% by weight of the cured film composition.

My invention relates to carbon electrical resistor compositions and to electrical resistor films produced therewith. More particularly, the invention relates to carbon film compositions consisting essentially of carbon, a re ducing oxide and resin, said compositions being applied to and cured on an insulating material to produce electrical resistors to electrical resistors so produced.

Presently, carbon film resistors are formed by either pyrolytically depositing the carbon from a gas onto a ceramic base or by coating an insulating base with a mixture of carbon particles in an insulating binder. These methods have been used for a long period of years, during which time various methods and techniques have been developed to improve the processes as well as the final products, and the advantages of each type of resistor formed thereby are well known. However, there still remains many inherent deficiencies in both types of resistors.

There may exist many advantages with carbon film resistors prepared by applying carbon paste consisting of carbon and resin to an insulating material and curing the same at a temperature depending on the resin. The main advantages are low cost and easy manufacture of resistors. It is also an advantage that the surface resistivity of films in a given thickness is reproducibily controlled by adding, to the carbon paste, oxides as a filler of the resin. The carbon film resistors made of carbon paste, however, have an inferiority in an electrical stability because carbon the more easily oxidizes and evaporates when the resistors are used in a high humidity ambient in connection with an electric field. For im provement of the stability, the resistors may be usually coated with a water proof material such as resin or glass. This procedure is not still satisfactory for preventing the oxidation of carbon and increases the production cost.

It is an object of this invention to provide carbon paste compositions characterized by a high electrical stability even when resistors made of said compositions are used in a high humidity ambient in connection with an electric field.

It is another object to provide carbon film resistor compositions having a desirable viscosity for coating.

It is a further object to provide electrical resistors characterized by a high reproducibility.

Other objects of the invention will appear hereinafter.

The resistor compositions of this invention may be prepared by mixing a finely divided metal oxide having a reducing power with carbon and resin in an inert solvent. The metal oxide is selected from the group consisting of Mn O Fe O Cu O and V 0 A combination of the above oxides also may be employed as a mixing agent.

For a further understanding of the nature and objects of the invention, references should be had to the following detailed description taken in connection with the accompanying drawings, in which:

FIG. 1 is a more or less diagrammatic View in elevation of a preferred embodiment of the invention which is suitable for use in a high resistance device;

FIG. 2 shows a convenient testing method of humidity load life of carbon film resistor compositions; and

FIG. 3 is a graphical illustration of humidity load life test of carbon film resistor compositions.

In FIG. 1, carbon film 1 comprising carbon, reducing oxide and resin is coated onto a ceramic core and removed locally in a spiral form by a well-known method for increasing the resistance of the film and effective length of carbon film depends upon the desired resistance. A lead 3 is welded to a cap 4 made of brass electroplated with nickel in a well-known method. When the resistor is used in a high humidity ambient in connection with an electric field, water is possibly absorbed on the surface of film 1 and space 2, and decomposed into hydrogen and oxygen by an electrolysis under the electric field. The electrolysis of absorbed water is strongly promoted at the space 2 because there appears a potential difference at both ends of the space 2. The carbon particles are oxidized by the generated oxygen and the resistance of film containing no reducing oxide particles markedly increases.

It has been discovered according to the present invention that the oxidation of carbon film is substantially prevented by mixing a reducing oxide with carbon. Any kind of finely divided oxide having a reducing power is satisfactory to accept the generated oxygen and protect the film. It is preferable that the oxide is selected from Mn O Fe O Cu O and V 0 in fine powder form, because these oxides are in lower valency states and are easily converted into higher valency states by their chemical reactions with the oxygen. Since the reaction rate increases and the paste characteristic improves with a decrease in the particle size of the additive oxides, control of the particle size is necessary to obtain the improved carbon film resistor. It is desirable to use said oxides in 0.5a to 50 average particle size. The most preferable average particle size is from 1,1 to 10 1,.

The electrical stability of carbon film resistors of the invention has a tendency to increase with an increase in the weight ratio of additive oxide to carbon. The high Weight ratio, however, results in a poor paste characteris tie and a poor reproducibility of electrical resistance. The following weight ratios of the metal oxides to carbon may be used:

Oxide: Preferable weight ratios Fe O 1.5 to 12 Mn Q 1.5 to 12 Cu O 2.0 to 10 V203 to The above mentioned oxides may be prepared by a well-known chemical technique or may be obtained from commercial reagents in chemical purity. The particle size of oxides may be controlled by changing a condition of chemical precipitation or by changing a decomposition temperature according to prior art.

The bonding resin used in the production of carbon film resistor compositions of the invention may be composed of any resin having a high electric resistivity such as phenol formaldehyde resin, melamine resin, urea resin, epoxy resin and their combinations.

The electrical resistance of the film and its reproducibility also are governed by the amount of resin. A high content of resin in the cured films results in a high electrical resistance and poor reproducibility of electrical properties and a low content of resin causes a weak bonding of carbon and metal oxide particles. Preferable weight ercentages of resin in the cured film compositions will be from 65% to 20% by weight.

From a consideration of the preferable weight ratios of reducing oxide powder to carbon powder and of the preferable weight percentages of resin, I have found that the operable weight percentages of carbon film resistor compositions are: 65 to 20 weight percent of resin, 14 to 6 weight percent of carbon film and 21 to 74 weight percent of finely divided reducing oxide selected from the group of Fe O Mn O Cu O and V and the preferable weight percentages are as follows; 65 to 20 weight percent of resin, 14 to 6 weight percent of carbon powder and 21 to 74 weight percent of Fe O powder; 65 to 20 weight percent of resin, 14 to 6 weight percent of carbon powder and 21 to 74 weight percent of Mn O powder; 65 to 20 weight percent of resin, 12 to 7 weight percent of carbon powder and 23 to 73 weight percent of Cu O; and 65 to 20 weight percent of resin, 12 to 7 weight percent of carbon powder and 23 to 73 weight percent of V 0 powder.

Carbon particles used in the production of carbon film are preferably composed of acetylene carbon black because its particles size is characterized by a uniform distribution of fine particles less than 0.1 Any other carbon powder used in a conventional carbon film resistor compositions may be employed for production of carbon film compositions of the invention. The uniformity is necessary to obtain an improved paint characteristic and a high reproducibility of electrical resistance of the film.

The carbon particles, oxide particles and resin in an inert solvent may be mixed in any manner, for example, in a mixing roller according to prior art. The solvent may vary widely in composition. A liquid which is inert to carbon and the oxide, dissolves the resin and evaporates after curing of resin, for example, benzyl alcohol, butyl Carbitol, methyl Carbitol and Tetralin is suitable.

The carbon paste of the invention may be applied onto a ceramic core by a stencil, spray or brush method and the content of solvent in the carbon paste compositions depends upon the coating method. For stencil screen application the viscosity of the paste must be controlled since it will affect the thickness of the prints and the resistance value of the resistors. The viscosity of the compositions may be adjusted so that the cured film is of the desired thickness. The preferable viscosity would be of an order of 500 to 5000 poises.

One method of testing the stability of a resistor with regard to humidity and the load applied to the resistor is known as the humidity load life test and comprises maintaining the resistor at a known humidity, and applying the load to the resistor which is maintained over a long period of time at a given temperature. This test, however, requires a long period of time and sometimes is not proper due to unexpected factors caused during the test.

Referring to FIG. 2, a new stability test has been developed wherein a carbon film resistor composition 5 and 6 is applied onto a ceramic plate 7 having high electrical resistance by screen printing and curing the composition in a well known manner. For a resistor having an area of 4 x 20 mm., the film thickness is adjusted to provide a resistance in the order of 10K!) to SOKQ. The two resistors, 5 and 6, of this size are electroded at both ends with silver paints 8, 9, .10 and 11 which are soldered to lead wires 12, 13, 14 and 15. The two films are spaced from each other by 2 mm. Paraffin 16 is placed on the films so as to partition the film which is subjected to a humidity load life test. The partitioned part 17 is in 30 mm. and 4 mm. A water drop of a suitable size is placed on the part 17 to establish the current circuit between the two films 5 and 6. A constant DC. voltage is applied to lead wires 12 and 14 thereby causing current flow through the partitioned part 17. During this current fiow, electrolysis of Water between the two films takes place markedly and the film of anode side is oxidized by the generated oxygen. Accordingly, the electrical resistance of the film on the anode side increases with the current flow. The stability of a resistor with humidity clearly relates to an electrical quantity necessary for obtaining a value of 10 of a variation ratio of initial electrical resistance of the film in anode to electrical resistance of the same film after actual period of current flowing. The electrical resistance of said film is checked occasionally under removal of water by a well known method. The electrical quantity is determined by an integration of flowing current with time and is a measure of the stability of a carbon film resistor with humidity-load life. A carbon film resistor is the more stable, said electrical quantity is the higher. For convenience, a ratio of said electrical quantity for a film comprising the reducing oxide to that of a film comprising no oxide is taken as a measure of the stability of carbon film resistor composition containing various metal oxides in various proportions. This ratio is named tentatively stability factor. The high stability factor means a high stability of carbon film resistor composition with humidity-load life.

The following examples are given to illustrate certain preferred details of the invention, it being understood that the details of the examples are not to be taken as in any way limiting the invention thereto.

Carbon film resistor compositions consisting of the following materials in the following weight parts listed in Tables I to IV are blended in benzyl alcohol as a solvent by using a mixing roller. An average particle size of reducing oxides is of an order of 1,44 to 10 Carbon film resistor compositions are presented by weight parts in Tables I to IV and weight parts of benzyl alcohol are added to the compositions to form a carbon film resistor paste. This paste is printed onto a ceramic substrate by a screen printing method and cured for 4 hours at C. for production of spaced band films described in the specification. Following the procedure described in specification, the stability factors are obtained. Tables I to 1V illustrate that the stability factors of carbon film resistor compositions are appreciably increased by adding Fe O Mn O Cu O or V 0 powder to carbon.

TABLE I Sample Phenol form Acetylene Stability number aldehyde black carbon F6304 factor resin TABLE II Sample Phenol form- Acetylene Stability number aldehyde black carbon Mn O; factor resin TABLE III Sample Phenol form Acetylene Stability number aldehyde black carbon Cu O factor resm Since a high stability factor is equivalent to high stability of carbon film resistor with humidity-load life it will be understood that the addition of R 0 Mn O Cu O or V 0 improves the stability of carbon film resistor with humidity-load life. A composition of samples 1, 4, 7, and 13 in the tables are applied to a ceramic core in mm. of length and '8 mm. of diameter by brushing technique in accordance with prior arts and cured at 150 C. for 4 hours. The surface of carbon film is helically grooved by about 0.2 mm of width for localized removal of carbon for obtaining of about 1 megohm of resistance. The cores provided with the grooved carbon film compositions are electroded with caps which are soldered to wires for production of resistors. These resistors are subjected to an usual humidity load life test at 25 C.

at 90% of relative humidity for a period of 2000 hours at their full rated electrical load. The results are shown in FIG. 3, wherein the stability is represented by where R0 is the initial resistance and R is the actual resistance during the test. FIG. 3 shows that the carbon film resistor compositions comprising Fe O Mn O Cu O or V 0 in accordance with the invention show a remarkably high stability with the humidity load life test. It will be seen that the stability of the carbon film resistor composition with Fe O Mn O Cu O or V 0 in the parts by weight in samples 4, 7, 10 and 13 in the tables is respectively thirteen, nine, five or four times as high as that of a composition without any reducing oxide.

Changes may be made with the scope and spirit of the claim and portions of improvements may be used without others.

What is claimed is:

1. A carbon film resistor composition adapted to be applied to and cured on a ceramic material to form humidity-load stable resistors consisting essentially of 65 weight percent to 20 weight percent of resin, 14 weight percent to 6 weight percent of carbon in fine powder form and 21 weight percent to 74 weight percent of finely divided Fe O powder.

2. A carbon film resistor composition adapted to be applied to and cured on a ceramic material to form humidity-load stable resistors consisting essentially of weight percent to 20 weight percent of resin, 14 weight percent to 6 weight percent of carbon in fine powder form and 21 weight percent to 74 weight percent finely divided M11 0 powder.

3. A carbon film resistor composition adapted to be applied to and cured on a ceramic material to form humidity-load stable resistors consisting essentially of 65 weight percent to 20 weight percent of resin, 12. weight percent to 7 weight percent of carbon in fine powder form and 23 weight percent to 73 weight percent of finely divided Cu O powder.

4. A carbon film resistor composition adapted to 'be applied to and cured on a ceramic material to form humidity-load stable resistors consisting essentially of 65 weight percent to 20 weight percent of resin, 12 weight percent to 7 Weight percent of carbon in fine powder form and 23 weight percent to 73 weight percent of finely divided V 0 powder.

5. An electrical resistor comprising a ceramic material containing on the surface thereof a carbon film resistor composition consisting essentially of 65% to 20% weight of resin, 14% to 6% weight of carbon powder and 21% to 74% weight of finely divided Fe O 6. An electrical resistor comprising a ceramic material containing on the surface thereof a carbon film resistor composition consisting essentially of 65% to 20% weight of resin, 14% to 6% weight of carbon powder and 21% to 74% weight of finely divided Mn O 7. An electrical resistor comprising a ceramic material containing on the surface thereof a carbon film resistor composition consisting essentially of 65 to 20% weight of resin, 12% to 7% weight of carbon powder and 23% to 73% Weight of finely divided Cu O.

8. An electrical resistor comprising a ceramic material containing on the surface thereof a carbon film resistor composition consisting essentially of '65 to 20% weight of resin, 12% to 7% weight of carbon powder and 23% to 73% weight of finely divided V 0 References Cited UNITED STATES PATENTS 11/1943 Duston -252- 511 LEON D. ROSDOL, Primary Examiner.

J. D. WELSH, Assistant Examiner. 

1. A CARBON FILM RESISTOR COMPOSITION ADAPTED TO BE APPLIED TO AND CURED ON A CERAMIC MATERIAL TO FORM HUMIDITY-LOAD STABLE RESISTORS CONSISTING ESSENTIALLY OF 65 WEIGHT PERCENT TO 20 WEIGHT PERCENT OF RESIN, 14 WEIGHT 